The invention is in the field of joint construction, and also the production of structural members from composites, and particularly composites produced by a pultrusion process.
Applicant is the inventor and the owner of U.S. Pat. No. 4,715,503 for an INTERLOCKING JOINT WINE RACK, U.S. Pat. No. 4,809,146 for an ENCLOSURE WITH INTERLOCKING FRAME JOINTS, U.S. Pat. No. 4,825,620 for a STRUCTURAL SUPPORT OF INTERLOCKING LATTICE CONSTRUCTION, and allowed patent applications Nos. 231,379 for a SUPPORT STAND and 541,547 for INTERLOCKING SUPPORT STRUCTURES. All of these disclosures relate to an interlocking joint construction technique that can be used with any kind of construction material such as wood, steel, concrete or composites to make three dimensional, three-member joints without the use of fasteners or cement.
Although very useful for joining wood, steel and concrete, these materials are by their nature not terribly difficult to join together using conventional nailing, bolting, bracketing and cementing techniques. However, this is not true of composite construction. Composites, as a general rule, do not lend themselves to conventional joining techniques. A straight, pultruded composite structural member may have extremely high tensile strength, but suffer from the inability to join it to other members in a strong, durable joint.
Composite pultrusions are made from one of several processes developed to produce structural members made from fibers, such as graphite, or fiberglass, and a resin such as polyester, vinyl ester, or epoxy, see U.S. Pat. No. 3,556,888 issued to W. B. Goldsworthy. The pultrusion process involves pulling a group of resin-saturated fibers through an extrusion (actually pultrusion) die directly analogous to extrusion processes. One significant feature of a fiberglass pultrusion is that it can have the same tensile strength as steel with one fifth the weight. Much of the strength is in the longitudinal direction (the direction of the pultrusion), but substantial progress is being made to improve the inner laminar shear characteristics by introducing cross-directional, or omni-directional fibers or fabric into the pultrusion for fiber glass pultrusions.
The applicant's joint system as disclosed in the above-referenced patents and patent applications has been modified to produce a very efficient and novel system of connectivity for fiber glass pultrusions. As evidenced by the Goldsworthy U.S. Pat. No. 3,556,888, fiber glass pultrusions have been in production for over 30 years and the process is well understood. Most products produced today are advantageous for use as a single pultrusion for use as axe and hammer handles, poles and the like, but the assembly of multiple pultrusions has been limited by the weakness which is characteristic of the joints.
Joining techniques as currently recommended by pultrusion manufacturers comprise the use of a combination of mechanical fasteners and adhesive bonding. One manufacturer illustrates the joining of structural pultrusions using a combination of bolts and epoxy in a very time-consuming process, producing a joint that has an allowable stress limit of 1000 psi, compared to the allowable stress limit of 30,000 psi. for the pultrusion member itself. The joint is thus only one thirtieth the strength of the member itself. As this illustrates, strength loss at the joints in multi-member composite pultrusion construction is not a minor problem, but one which makes the use of composites impractical in an enormous range of structural implementations.
Obviously a simple, strong and effective system for joining these remarkable structural members is not evident in the connective technology developed to data.
Although this technology can be applied in hundreds of different fields, one area of particular interest in this disclosure is in the electrical field, and particularly high voltage, high power applications such as power transmission lines and towers. The advantages of high dielectric strength of fiber glass composites has allowed these materials to find their way into many electric utility applications. The high dielectric strength properties produces electrical insulative characteristics. Some applications within the electric utility industry for fiberglass pultrusions include ladders, switch lanyard poles, hot line equipment for linemen, structural interior rod for insulators, and booms for maintenance hoists known as "cherry pickers", to mention a few.
However, composites have not been used significantly in large structures such as power transmission line towers. Steel, aluminum and wood have been the only choices available to that industry for these structures. Wood (treated with creosote and other preservatives) has been a standard material, but eventually falls victim to decay or damage from birds and insects. Steel and aluminum have been used predominantly for lattice-type power transmission towers and substation structures. Although they are high-strength, the electrical conducting features of these metals make them the most imperfect and inadequate choice, given the availability of the materials of instant invention.
But again, one single drawback in the material properties of fiberglass pultrusions has prevented them from being used in many larger electrical utility structures requiring connecting of multiple members, which is, fiberglass putlrusions have low bearing strength. This has resulted in very poor performance of multiple member composite structures using conventional fastening and connecting techniques. Composite bolts have been developed in an attempt to overcome this drawback (and keep the joint system fully insulated and corrosion resistant) but these bolts have poor thread performance. Most systems have required fastening plus adhesive bonding at any pultrusion joint and still these joints have been significantly weaker than the pultrusion member itself.
Thus, although these materials have been used in electrical utilities for over 30 years, the absence of a good connective technology has prevented their use in large structures such as high voltage overhead power transmission towers and structural substation supports. The use of the applicant's connective technology with fiberglass pultrusion composites will offer many advantages to the world's electric utilities, well into the next century.